The present invention relates to a controllable liquid crystal matrix (patternable) mask for controlling ultraviolet laser energy, and a system which involves the use of the controllable liquid crystal matrix mask for use in, for example, an ophthalmic surgical system such as an excimer laser system for contouring the cornea through controlled ablation of the cornea with penetration into the stroma and volumetric removal of corneal tissue whereby the ablated corneal surface is characterized by a sculptured new curvature having improved optical properties.
Various surgical techniques for reprofiling of the corneal surface have been proposed as described in, for example, L""Esperance, U.S. Pat. No. 4,732,14; J. T.Lin, U.S. Pat. No. 5,520,679; David F Muller, U.S. Pat. No. 4,856,513; Kristian Hohia, U.S. Pat. No. 5,520,679. Each of these patents is incorporated by reference herein.
In practice there are two basic techniques to ablate and remove a set volume of tissue in the cornea and they are:
1) a scanning technique that uses a small flying laser spot between 1-2 mm in diameter and requires thousands of pulses to do the surgery; and
2) a large spot beam technique wherein a laser beam of around 8 mm in cross-section is used in conjunction with an erodible mask or a moving, blocking mask to ablate and which generally requires, on average, a few hundred pulses to achieve the desired ablation.
The main advantage of a flying laser spot technique is the ability to readily execute irregular patterns. However, the flying laser spot technique suffers from the drawback of generally requiring longer surgical times to execute the desired ablation pattern both from the standpoint of the number of pulses required and the overlapping requirement to ensure coverage of the ablated area. Any increase in the length of time required to carry out the laser ablation process can lead to longer corneal exposure time and corresponding medical concerns such as cornea dehydration which can lead to poorer healing, poorer visual acuity and, in general, longer post operative recovery times. A longer time period in which a laser is operated per patient also leads to a decrease in the useful life of the laser and an increase in service requirements. In addition to the time delay associated with a flying spot laser technique, there is also the problem of ridges and valleys being formed due to overlapping that will prevent a highly smooth and polished ablation. Flying laser spot techniques place greater stress on the laser cavity and the optical train components due to high repetition and rate requirements.
A large beam spot system does allow for more rapid application of the desired energy (including the avoidance of the degree of overlapping involved in a randomly or non-randomly applied, partially overlapping flying spot system) and typically places less strain on the laser equipment, but does not have the irregular pattern versatility provided with a flying spot system. Also, if mechanically moving components are relied upon as the means for blocking or allowing through the laser beam (e.g., an iris or rotating or sliding aperture plate), then the problems of mechanical wear, potential jamming or breakdown arise.
Attempts have been made to provide masks that operate with a large beam application including EP 0 417 952 to Rose et al. which uses a stacked set of binarily weighted masks (A to H) on a cornea intended for sculpturing. A proposed system such as this one suffers from a variety of drawbacks such as the time consumption involved in stacking and developing the mask sets and the increased potential for error brought about by a system using so many different stacked mask components.
Another example of the use of large beam application is found in U.S. Pat. No. 4,994,058 which describes a supported or contact type mask presenting a predetermined resistance to the beam such as through use of a different height erodible mask or by varying the composition of the plastic material making up the mask. A mask arrangement such as this avoids the complications of a scanning laser, but is very limited from the standpoint of having to prepare a new mask for each patient and, with respect to an erodible mask, not having the benefit of being able to test the pattern on a test strip or the like without destroying the mask. In use of an erodible mask there is also heavy reliance on chosen material consistency.
The present invention is directed at providing a controllable, patternable liquid crystal mask that is particularly well suited for use with ultraviolet electromagnetic energy utilized in an ophthamological surgical procedure such as a photo refractive keratectomy (PRK); photo therapeutic karatectomy (PTK); and a laser in-situ keratomileusis (LASIK) surgical procedure for resculpturing the exposed cornea of an eye. Accordingly, the present invention is directed at providing a controllable patternable mask system, an ophthalmic laser surgery system with said controllable mask system, and a method of using the same. The apparatus and method of the present invention is particularly well suited for ablating a corneal surface using a laser in the ultraviolet spectrum to achieve extremely smooth and precise ablation surfaces with a mask system that can be repeatedly used for different patient ablation requirements. The mask is used with a large beam spot (e.g., 6-8 mm) which covers the entire projected surface on a cornea and thus avoids the time delays associated with a flying spot beam as well as ridge and valley formation. In addition to multi-patient use, the present invention avoids the delays associated with the prior art with respect to forming or assembling the mask. Moreover, the present invention provides a system which can achieve repeated high precision or registration between the planned or predetermined ablation volume to be removed and the actual ablation volume removed so as to better enable a surgeon to achieve desired levels of eyesight corrections. Also, the volumetric ablation patterns to be removed can involve highly irregular or regular configurations, as an object of the present invention is to provide a system which facilitates the execution of a surgical procedure on highly irregular individual customized patterns on a patient""s cornea in addition to more regular volumetric patterns.
Furthermore, the arrangement of the present invention provides a system that can achieve a customized volumetric ablation pattern based on, for example, a stored, ophthamological patient data set (e.g., an ablation data set as described in U.S. patent application Ser. No. 09/267,926 filed Mar. 10, 1999 by Dr. Luis Ruiz which application is incorporated herein) which data set is developed by a measuring instrument such as a topographer or aberrometer and, under the present invention, is provided to a processor for input to the mask system via a digital interface, for example. The matrix mask system of the present invention includes an active liquid crystal matrix mask which receives expanded, polarized laser energy and, based on the processed patient data, controls the energy pattern that exits in the mask for sculpturing the desired ablation pattern on the cornea.
Under the present invention, the ablation pattern data set is processed by a processor such as the laser""s main computer which communicates with the mask via an interface or the like to provide corresponding commands to activate the matrix mask. For example, in one embodiment of the invention, the transmission pixel pattern of the mask is individually controlled by the computer and is synchronized with the pulse rate of the main excimer laser. In this way, any regular or irregular ablation volume can be removed by ablating with each pulse of the large beam a set depth (based to the laser characteristics such as laser energy and density) corresponding with the matrix pattern set for that pulse. By changing the mask""s matrix pixel pattern with each large beam pulse a different ablation configuration (or the same) can be removed to achieve the desired total pattern, volume and depth.
With an excimer laser working in the ultraviolet energy spectrum, the components of the optical train are subjected to relatively high energy flux levels. Under the present invention, the large beam is preferably expanded with a beam expander or the like positioned upstream from the active matrix mask to lower the energy density per area received by the pixels of the mask. This will avoid premature destruction of the mask. The non-blocked, transmitted portions of the large beam containing the latent image determined by the active mask are then compressed with a focusing lens or the like to achieve the desired ablation profile.
Prior to reaching the liquid crystal material of the liquid crystal active mask, the laser beam is first polarized with a polarizer and, after exiting the liquid crystal material, the beam with the latent image passes through a second polarizer also known as an analyzer to the ablation plane or surface. The position of the expander/collimator can be either before or after the first polarizer, with the same being true for the relationship between the focusing lens and second polarizer.
Thus, the present invention preferably features a controlled ablation of the cornea, using ultraviolet laser radiation to achieve a sculpturing action derived from a computer driving a digital programmable distribution of excimer flux density across an ultraviolet computer controllable liquid crystal matrix mask as to achieve a desired volume and shape of the ablation for the correction of the curvature of the cornea.
The computer controllable liquid crystal matrix mask is based on a matrix array (matrix is used in a broad sense to involve any pattern suited for forming a desired ablation pattern on the ablation plane or surface) of individual optical liquid crystal pixel cells with optical states between fully transparent to fully opaque or mirror like to the UV light. (Preferably, for high resolution purposes, each cell is individually controlled although groups of interlinked pixels could be controlled by the processor as a group unit to present a combined transmission state. Hence, the term xe2x80x9cpixelxe2x80x9d or xe2x80x9cpixel cellsxe2x80x9d is used herein unless otherwise indicated to include single or groups of similarly controlled pixel element groups.) As noted, the ultraviolet computer controllable liquid crystal matrix mask is based on transmission of polarized light through the mask device, the transmission of polarized light through each individual pixel is preferably controlled by an electrical voltage and a time duty cycle. In one embodiment of the invention the voltage is switched or adjusted to have the pixels either in a fully transparent or fully blocking mode with the pixel switching timing being synchronized with the laser pulse rate. In addition thereto, the frequency duty cycle per pixel can be adjusted or varied with relation to the laser pulse cycle of the main laser to achieve multiple on and off states of transmission during the main laser pulse cycle so as to control the maximum state of ablation during that main pulse cycle.
Preferably the active matrix liquid crystal mask is based on the twisted nematic (TN) natural physical effect of liquid crystals for controlling light transmission. The principle of the active matrix mask device of the present invention is as follows: Unpolarized excimer light enters a first polarizer and emerges polarized, the polarized light (UV electromagnetic radiation) is sent into the liquid crystal mask which comprises many pixels. When no voltage is applied to a pixel, the polarization vector of the incoming light is rotated a quarter turn/90xc2x0 by the liquid crystal molecules through the TN natural physical effect. A second polarizer (or analyzer) placed at the output side of the liquid crystal cell is used to transmit either the light (normally on) or reject the light (normally off). When a proper voltage is applied to the electrodes of the pixel, the TN effect disappears (as the liquid crystals tend to orient with the electric field resulting in a loss of their natural twisting function) and the polarization vector of the polarized light is unchanged. The light will therefore be rejected (off) or transmitted (on) by the second output polarizer, depending on whether the device is normally on or off, respectively.
Accordingly, the crystal molecules within the pixel cells act as an optical wave guide by rotating the polarization vector of the UV light which is to reach the second polarizer. If the second polarizer is set to have a polarization vector coincident with that of the twisted polarized UV light, then the cell will be xe2x80x9conxe2x80x9d when no electric field is imposed as the cell allows the UV light to transmit through it to the desired ablation surface. In this state the liquid crystal pixel cell is transparent since the second polarizer has a vector coincident with the twisted polarized UV light. With this arrangement, the passage of non-twisted UV polarized light due to the generation of an electric field will be blocked and the cell will appear dark.
If, instead, the second polarizer is arranged as to be non-coincident with the twisted polarizer UV light vector, then the second polarizer will act to prevent transmission of the twisted polarized UV light and will be xe2x80x9coffxe2x80x9d and the cell will appear non-transparent. With this arrangement, the second polarizer is arranged to be coincident with the untwisted polarized UV light (made possible by an electric field generation) such that the un-twisted light energy passes through the second polarizer for transmission to the ablation surface) when an energy field is generated. Each pixel can be turned on or off independently by the active control of the voltage across it.
A preferred embodiment of the present invention features a mask system for controlling the transmission of ultraviolet electromagnetic radiation (UV) that comprises a first UV grade polarizer, a liquid crystal matrix mask having a plurality of individual pixel cells with individually adjustable states of transmission with respect to polarized UV electromagnetic radiation received from said first UV grade polarizer, and a second UV grade polarizer also known as the analyzer positioned downstream for receipt of polarized UV electromagnetic radiation transmitted by the liquid crystal matrix mask. The active mask of the mask system is preferably in communication with control means, such as a processor and a digital interface or the like, for controlling relative transmission states amongst the individual pixels in the matrix array.
The mask system of the present invention also preferably includes a beam expander/collimator assembly which is positioned in line with the UV electromagnetic radiation travel path and is positioned upstream with respect to the liquid crystal material of the liquid crystal mask. The beam expander/collimator is also preferably positioned downstream of said first polarizer and upstream of the liquid crystal material with respect to UV electromagnetic radiation travel. The mask system also preferably comprises a focusing lens that is positioned downstream of the liquid crystal material and upstream of the second polarizer, again with respect to UV electromagnetic radiation travel. In addition, for desired laser designs for use in the present invention it is further preferable to have a turning mirror positioned within the optical path leading to the active mask and preferably between the expander/collimator assembly and the active mask with respect to laser beam travel.
As noted, in a preferred embodiment, the liquid crystal mask includes a twisted nematics liquid crystal material. In addition, the first and second polarizers are preferably physically separated from the liquid crystal mask. This mask embodiment also comprises an inner and outer UV grade substrate (e.g., UV grade synthetic fused silica or sapphire) with deposited electrode material on an inner side so as to provide a sandwiching effect with respect to the liquid crystal material in between the two electrode devices. Also, the first and second polarizers are preferably designed for peak polarization of UV electromagnetic radiation at 193 nm.
The aforementioned processor interfaced with the mask for controlling relative transmission states between said individual pixel cells preferably includes means for shifting a pattern formed in said array to have the shifted pattern position correspond with a shift in position of a substrate to receive UV electromagnetic radiation transmitted by the mask. As one example, the means for shifting the pattern works in conjunction with an eye tracker system so as to shift the pixel pattern in conjunction with a detected shift in the cornea position of the eye detected by an eye tracker.
The mask system of the present invention preferably features a mask with a pixel array having at least 512xc3x97512 resolution with each pixel size being 125xcexc or less. Even more preferably, the pixel array is at least 1024xc3x971024 in number and the pixel size 100xcexc or less.
In one embodiment of the invention, the individually adjustable states of transmission are limited to fully transmitting xe2x80x9conxe2x80x9d and fully blocking xe2x80x9coff.xe2x80x9d In another embodiment of the invention, the mask, which is designed for receipt of laser pulses, further comprises pixel switching timing means for controlling the timing of pixel switching between different on and off states (when desired). In one embodiment, switching takes place in a one-to-one relationship with respect to laser pulses received by the mask. That is, at some time between each pulse, pixel switching activity takes place to alter the pattern presented to the incoming beam of the next pulse.
The mask system of the present invention can further include a pixel switching timing means for timing individual pixel switching between different states and which sets a duty cycle for individual pixels at a duty cycle of from 0% (off for full pulse transmission period) to 100% (on for full pulse transmission period) with at least one or more set pixel duty cycle(s) being at an intermediate value(s) falling between 0 to 100% with respect to the pulse cycle of the main laser beam.
The present invention is also directed at a laser system for ophthamological surgery which comprises a laser, and a liquid crystal mask having a plurality of individual pixel cells positioned for receiving a laser beam output by the laser. The individual pixel cells of the mask have adjustable transmission states for forming a transmission pattern with respect to the laser beam output by the laser. In an embodiment of the invention, a first UV grade polarizer is positioned upstream (with respect to laser beam travel) to the pixels while a second polarizer is positioned downstream (with respect to laser beam travel) from the pixels. In this embodiment, a twisted nematic liquid crystal material is also preferred. The first and second polarizers are provided with a common polar vector direction in one embodiment, while in an alternate embodiment the first and second polarizers have a non-common or shifted polar vector direction.
The laser system of the present invention further preferably has an expander/collimator assembly that is positioned downstream with respect to said first polarizer and upstream with respect to the liquid crystal material of said matrix, together with a focusing member positioned downstream from the liquid crystal mask with the mask having first and second substrates of a UV grade material, and together with a turning mirror for redirecting an expanded large spot beam to the matrix in a perpendicular relationship.
The focusing member is designed to focus a latent image pattern formed by the mask onto a projected surface wherein transmitted UV energy of adjacent transmitting pixels is compressed into a minimum overlap relationship (e.g., one involving less than 5% or and more preferably less than 1% overlap of the adjacent ablated regions generated by the pixel cells) or a minimized spacing relationship (e.g., one involving less that 5% spread apart and more preferably less than 1% spread), or an essentially abutting relationship (of xc2x10.75% overlap/spacing apart).
The laser system of the present invention preferably includes directing means for directing pixel transmission state controls to said mask wherein said directing means includes a processor and an interface linking said processor and mask. The processing means preferably further comprises means for processing acquired volumetric ablation volume data and basing pixel control outputs to said matrix on the processed acquired volumetric ablation volume data. The laser system also further includes means for acquiring three dimensional volumetric ablation pattern data such as means for receiving corneal ablation data based on topographical and/or aberrometer measurements.
In a preferred embodiment of the invention, the processor further includes means for segmenting acquired corneal volumetric ablation pattern data into a plurality of matrix segments and means for determining a desired individual pixel transmission status arrangement for respective segments of said acquired volumetric ablation pattern. Preferably all processor functions are handled by a central computer in the laser system of the present invention although sub-processor(s) or sub-processor group(s) are also suited for use in the present invention. The system of the present invention also preferably comprises means for sequentially changing a pixel array pattern based on respective, determined pixel transmission statuses for the individual ablation segments stored in a memory stack, such as a top to bottom or bottom to top memory stack sequence with respect to the ablation segments derived from the acquired volumetric ablation pattern.
As the liquid crystal material matrix features a deposited electrode plating or the like on the UV grade substrates, the processor of the laser system is in communication with the mask via an interface device such as a digital interface device for converting digital information to individual pixel voltage signals. The laser system of the present invention preferably also includes means for determining a transmission switching rate for individual pixels of said array based on a duty cycle correlated with a laser pulse period of the laser. For example, individual pixels of the mask are assigned one of two states, either a duty cycle of 0% (no transmission) state during the pulse period state or a 100% (full transmission) state during the pulse period state. In an alternate embodiment, at least some of the individual pixels of the mask are assigned a duty cycle that is intermediate the 0% (no-transmission) state during the pulse period and a 100% (full transmission) state during the pulse period.
A preferred embodiment of the laser system further includes an eye tracker and a processing device which is in communication with both said eye tracker and said mask, and said processing device includes means for implementing a switch or change over in a transmission state of individual pixel cells to shift a pattern defined by pixel cells in said mask from a first pattern position to a second pattern position in correspondence with a shift in location of a projected surface of an eye being monitored by said eye tracker. A similar means can be provided for monitoring and adjusting pixel positions for different types of substrates other than an eye which are subject to movement between laser pulses or mask refreshings.
The present invention is also directed at a method for ablating an eye cornea that comprises directing a polarized laser beam to a liquid crystal mask having a plurality of pixels with individually controllable pixel states, and directing xe2x80x9cthroughxe2x80x9d mask transmitted portions of the laser beam onto a corneal surface for sculpturing the corneal. The method of the present invention further includes altering a first pattern defined by said pixels into a second pattern between pulses of the laser to alter the ablation pattern on the cornea. A preferred method of the invention further comprises processing acquired volumetric ablation pattern data to provide pixel transmission state control signals to the individual pixels of the mask. Moreover, the present invention further comprises a method of monitoring a projected surface of the substrate to be ablated with means for monitoring and, with information obtained from the means for monitoring, shifting the pattern formed in the mask to compensate for a shifting in position of the projected surface prior to the next ablation energy application.